A liquid crystal display device characterized by extremely small power consumption has been widely used as an image display device mounted on an image device of an information device such as a notebook-type personal computer, a word processor and the like, or as an image device of a video device such as a portable television, a video movie, a car navigation system and the like, for securing sufficient operating time of the image device, because these image devices are often driven by built-in primary or secondary battery. As used herein, the liquid crystal display device refers to a device comprising, for example, a liquid crystal display panel structured such that two transparent glass substrates on which driving signal supplying electrodes formed by electrically conductive transparent thin films, alignment layers and the like are stacked, are superposed on each other with a predetermined spacing so that the surfaces of the stack are opposed to each other, and sealed after liquid crystal is filled therebetween, and polarizers are provided on outside of both the glass substrates, and configured to display an image by variation of transmissivity of light according to a driving signal inputted to the driving signal supplying electrode, a lighting unit disposed behind the liquid crystal display panel for supplying light to the liquid crystal display panel, and a circuit board and the like for driving the liquid crystal display panel.
FIG. 8 is a cross-sectional view showing an example of the liquid crystal display device comprising a conventional edge light type lighting unit. As a matter of convenience, a direction of the liquid crystal display device is shown as in the drawing. A liquid crystal display device L comprises a liquid crystal display panel 1 disposed such that a display surface 1a on which characters and images are displayed faces forward, front side frames FC formed by molding metal plates having a constant thickness into L-shape and disposed to cover a non-display display region of the display surface 1a of the liquid crystal display panel 1 with a predetermined spacing between the same and the liquid crystal display panel 1 and to form upper and lower surfaces of the end face of the liquid crystal display device L, a lighting unit UT disposed behind the liquid crystal display panel 1 for supplying light to the liquid crystal display panel, and a rear cover RC disposed to cover an entire rear surface of the lighting unit UT.
Herein, the lighting unit UT comprises a light guiding plate 3 formed by a transparent plate made of synthetic resin such as acrylic for guiding light entering from an end face thereof in a direction parallel to a principal surface thereof and emanating the light from the principal surface thereof, light sources 2 such as fluorescent discharge tubes disposed in the vicinity of end faces 3a of the light guiding plate 3 in a direction substantially parallel to the end faces 3a along the end faces 3a, reflectors 4 covering the light sources 2 in U-shape over a substantially entire length thereof, a reflecting sheet 5 formed by a white synthetic resinous sheet or the like having a high reflectivity for reflecting the light emanated from a rear principal surface of the light guiding plate 3 toward the light guiding plate 3 again, and housings 10 made of policarbonate resin or the like for supporting the reflectors 4 and the light guiding plate 3 at upper, lower, and front surfaces of the reflectors 4 and an outer peripheral portion of a front principal surface of the light guiding plate 3, fixing the liquid crystal display panel 1 and the light guiding plate 3 with a predetermined spacing, and fixing front side frames FC to predetermined positions so as to form gaps 7a. 
In the liquid crystal display device L structured as described above, light emitted from the light source 2 is efficiently introduced into the end face 3a of the light guiding plate 3 by means of the reflector 4, and the light introduced into the end face 3a of the light guiding plate 3 is guided in the direction away from the light source 2 by means of the light guiding plate 3, and uniformly emanated from a front side of the light guiding plate 3 by being reflected by means of the reflecting sheet 5, and the emanated light enters the liquid crystal display panel 1. And, images are displayed on the liquid crystal display panel 1 by variation of transmissivity of light according to the driving signal inputted to the driving signal supplying electrodes.
In a meanwhile, recently, the liquid crystal display device mounted on the image device so as to obtain a light and thin image device has been required to be light and thin. And, the liquid crystal display device used in the image device such as a monitor is required to obtain high luminance in addition to lightness and thinness. Furthermore, a screen thereof becomes larger. In response to such a trend, a liquid crystal display device using the edge light type lighting unit in which a tubular light source such as a fluorescent discharge tube is disposed on an end face thereof, which is suitable for obtaining lightness and thinness, high luminance, and a large screen, is widely used.
On the other hand, the liquid crystal display device is characterized in that light emission intensity of the light source lowers with elapse of time and a lifetime of the light source becomes short when a temperature in the vicinity of the light source becomes high by heat generated from the light source located within the lighting unit. Therefore, it is required to cool down the vicinity of the light source, or to secure a path for discharging the heat in the vicinity of the light source so as to hold a luminance of the liquid crystal display device constant during operation of the device, and to prevent occurrence of negative effects on the lifetime of the light source.
However, in the liquid crystal display device using the lighting unit of the conventional edge light type, since the light source is covered with the reflector, the housing, the rear cover and the like, there is a problem that the temperature in the vicinity of the light source becomes high by heat generated from the light source during operation of the device. And, if a cooling device is provided in the vicinity of the light source for cooling down the vicinity of the light source, a size of the liquid crystal display device becomes large with an additional volume of the cooling device, so that it becomes difficult to obtain the light and thin image device. Furthermore, since power consumption of the entire image device increases by providing the cooling device, a load on built-in primary or secondary battery increases, thereby causing a problem that securement of sufficient operating time of image device, which is an original object, is inhibited.